Scanning probe microscopes operate by scanning a probe over a surface that is to be examined, typically in a raster pattern. One type of scanning probe microscope is an atomic force microscope, in which the probe consists of a cantilever with a sharp tip located near its free end. As the cantilever is scanned, the tip interacts with the surface. This in turn causes physical changes in the cantilever which are detected and used to generate a representation of the surface, often down to an atomic scale. Other types of scanning probe microscopes include magnetic force microscopes and electric force microscopes, which detect phenomena other than the topography of a sample.
An atomic force microscope can operate in several modes. In the "contact" or "constant force" mode, the tip is maintained in contact with the surface. As the tip encounters topographical features of the surface, the cantilever is deflected or bent. These deflections are detected, and by means of a feedback system the distance between the cantilever and the surface is controlled so as to maintain a constant force between the tip and the surface. In the feedback system, a signal representative of the deflection of the cantilever is compared against a reference voltage to produce an error signal. Using the feedback electronics to hold the error signal to zero, an output is generated which both holds the error signal to zero by changing the tip-sample spacing, and generates a representation of the surface.
In the "dynamic" or "non-contact" mode the tip is brought very close to the surface, and the cantilever is vibrated at a frequency which is close to its resonant frequency. As the cantilever is scanned, the distance between the tip and the features of the surface varies. This in turn causes the gradient of the Van der Waals and other forces between the tip and the surface to change. Resulting changes in the vibrational amplitude, frequency or phase of the cantilever are detected, and again the distance between the cantilever and the surface is controlled by a feedback system to maintain the tip-sample separation at a constant.
The "cyclical" or "intermittent" contact" mode, described in U.S. Pat. Nos. 5,266,801 and 5,308,974, is somewhat similar to the dynamic mode, but the tip is allowed to strike the surface as the cantilever vibrates.
The separation between the cantilever and the sample surface is normally controlled by means of a piezoelectric tube on which the sample platform is mounted. The piezoelectric tube is part of the feedback system mentioned above. The output of the cantilever deflection detector is delivered to an input of the piezoelectric tube via the feedback electronics. Variations in this signal cause the tube to expand or contract along its axis and thereby adjust the position of the sample in relation to the cantilever in a direction normal to the surface of the sample. Alternatively, the cantilever can be mounted on the piezoelectric tube.
The ability of an atomic force microscope to generate images rapidly depends on the speed at which the cantilever is scanned over the sample. Typical scan speeds are in the range of 10 to 100 .mu.m/sec, which means that images take several minutes to generate. At speeds greater than this level, the cantilever, as it interacts with features on the sample surface, begins to interact at frequencies that approach the resonant frequency of the piezoelectric tube (typically between 2 and 10 kHz). When the piezoelectric tube moves into resonance, the feedback system used to maintain a constant tip-sample force (or spacing, if the microscope is operating in the dynamic mode) does not function properly.
By using the principles of this invention, the scan speed attainable by the cantilever can be significantly increased.